Our hypothesis is that viral infections have strong and diverging effects on different Treg populations. Our preliminary data indicate that Foxp3+ Tregs (adaptive or naturally occurring) and IL-10+ Tregs are affected by viral infections. The underlying mechanisms are proposed to be related to the modulation of APCs, in particular interferons and TLRs, resulting in strong effects on Treg activation, stability, which could be reflected in changes in the DNA methylation status ofthe Foxp3 promoter, and cytokine production, reflected in changes in the overall pattern of epigenetic markers in the genome. Clearly defining the underlying mechanisms and their precise effects will allow us not only to better understand the response of the immune system to viral infections, but to devise better strategies to induce Tregs for the treatment of chronic inflammatory disorders such as IBD, T1D and asthma. Aim 1: Do acute or chronic viral infections destabilize Foxp3 and/or IL-10 expression and Treg activity in vitro or in vivo? Using Foxp3 and IL-10 gene reporter mice, we will test our hypothesis that Foxp3 expression and IL-10 production are modulated during viral infection, and that this occurs differentially depending on whether the infection is acute (LCMV Armstrong) or chronic (LCMV Clone 13). We will compare effector functions of Tregs before, during, and after viral infections. Aim 2: How do Tregs influence the outcome of viral infections, autoimmunity, and asthma? Here we will test our hypothesis that different Treg types, in spite of their efficacy, are not comparable in terms of outcome and function in particular disease situations in vivo. Aim 3: How do viral infections affect Tregs stability and function mechanistically? Preliminary data indicate that Treg function is affected via APCs rather than a direct influence of viral infections on Tregs. In addition TLR2 appears to modulate and eventually expand and invigorate Foxp3+ Tregs. We propose that players in the innate immune system,such as DCs and TLRs, along with endogenous ligands, such as heat shock proteins (HSPs), play an important role in determining the outcome.